Opensky

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groovy

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OpenSky and/or OpenSky2... Pennsylvania state police use it, and as another poster mentioned(I think) some systems in Las Vegas, and other sporadic systems. Would be nice to add it to DSD.

First-
Do we have the spec of the protocol, or know where/how to get it?
Failing that, do we have an idea of the basis of the protocol to reverse engineer it? Is it based enough on CDPD or something else existing to be able to figure it out?
Do we know the exact codec? It supposedly fits into 4 TDMA slots, so the bitrate is perhaps lower than the 3600 rate codecs used by D-STAR? (Opensky2 uses the AMBE+2 codec we use for DMR/etc, perhaps opensky uses the older AMBE+ codec like D-STAR?)

In any case, it is more a data protocol that happens to be carrying voice than a voice protocol. This makes the treatment more complex than most of the DSD implementations. But I think if we put our collective heads together, we might be able to get this working in DSD...

Some references:
SkyCrack - HackerspaceWiki
PENNSYLVANIA STATE POLICE RADIO IN WESTMORELAND COUNTY, PA
OpenSky - The RadioReference Wiki
Talk:OpenSky - The RadioReference Wiki
http://forums.radioreference.com/harris-ma-com-ge-ericsson-forum/34946-opensky-cdpd.html
 

Radioman96p71

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I think the major hurdle is going to be encryption. It is my understanding from reading about the systems that they all run at least AES128 on all traffic, with AES256 optional.

The other problem is the entire system is designed to be IP based and packet routed, not circuit. That isn't a huge deal over the RF link because they should all be 'in order' somewhat, but because it is designed to operate more like a cell phone network, calls may not stay on the same tower for very long as users roam about.

I'm curious to see how this plays out, I don't have anything like that near me but OS seems to be the one system that hasn't been broken. I would venture a guess that it will be similar to the TETRA systems over the pond that are secured from end to end, so you can decode talkgroups and other similar info but all traffic is locked down.
 

SCPD

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From pastebin ...

Here's a short update on decoding the OpenSky 2 format. This is the narrow 9600 bps (4800 baud) version. The waveform is essentially the same as P25 C4FM.

Harris PSPC

Folding the data 560, 280, and 140 symbol intervals shows high repetition of data. During idle times it is hard to distinguish invariant data (like sync bits) from idle frame data.

The data is broken up into frames of 280 symbols. Each symbol is two bits. Each frame starts with a 24 symbol frame synchronization pattern followed by 256 symbols of data (64 bytes). The two frame synchronization patterns alternate one after the other. For the patterns below, each digit represents a symbol where 1 indicates a positive arc, 0 indicates a negative arc. If you prefer using +3/+1/-1/-3 with four level symbols then 1=+3 and 0=-3.

011010110001111101011111
011010110101010101111111

Each frame often appears to repeat itself. The first 128 symbols will be equal to or very similar to the second 128 symbols. It is unclear whether each frame is a time slot or each frame carries two time slots.
http://pastebin.com/raw.php?i=rA1yb7v8
 
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